Method for making secure a session with data processing means under the control of several entities

ABSTRACT

A method for securing the execution of a session with a data processor, such as a smart card, under the control of at least two entities, such as servers. Session numbers and session keys are transmitted to the entities. The session number and key are applied to an algorithm in the data processor and the respective entity to produce a result and signature. The results and the signatures are transmitted to the data processor. A session corresponding to the results from the data processor is executed when the signatures are identical to the results. In another embodiment, one of the entities receives a delegation of a third entity to authorize execution of the session.

This disclosure is based upon French Application No. 00/10025, filed onJul. 28, 2000 and International Application No. PCT/FR01/02354, filedJul. 26, 2001, the contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention concerns in general terms the making secure of theexecution of a session with a data processing means under the control offirst and second electronic entities.

For example, the data processing means is a multi-application smart cardin which some resources must be accessible provided that at least twoentities give authorisation to access this resource. This is because itis sometimes advantageous to make the writing in a file of a smart cardor in more practical terms the debiting of an account in a card of theelectronic purse type dependent on the authorisation of two electronicentities, such as bank and dispenser servers.

The present invention aims precisely to make secure the triggering of asession in the processing means, such as a smart card, under the controlof at least two electronic entities.

SUMMARY OF THE INVENTION

To this end, a method for making secure the execution of a session witha data processing means under the control of at least two electronicentities is characterised in that it comprises the following steps of:

-   -   transmitting session numbers and session keys from the        processing means respectively to the entities,    -   applying the respective session number and the respective        session key to a respective security algorithm in the processing        means and the respective entity in order to produce a respective        result and a respective signature,    -   transmitting the respective session number and the respective        signature from the respective entity to the processing means,        and    -   executing the session from the processing means when the        signatures are respectively identical to the results.

So that the processing means can be assured that the execution of therequested session does indeed correspond to the session numbertransmitted initially, the respective results are written in memory inthe processing means respectively corresponding to the respectivesession numbers to be transmitted to the entities and are read incorrespondence with the respective session numbers transmitted by theentities to the processing means before being compared with therespective signatures.

In practice, each of the entities transmits to the processing meansrespective data with the respective session number and the respectivesignature. The data contain an agreement or refusal to execute thesession. Thus the session is executed if in addition the processingmeans detects in each of the data an acceptance of the session by therespective entity.

According to a second embodiment, the session is executed provided thatone of the said at least two entities has received respectively adelegation for the execution of a session by a third entity. In thissecond embodiment, the method comprises the following steps of:

-   -   transmitting respective delegation information in favour of one        of the said at least two entities from a third electronic entity        to the processing means,    -   transmitting a session number, which is identical to the        respective session numbers, and a third session key from the        processing means to the third predetermined entity,    -   retransmitting the session number and the third session key by        the third entity to the said one entity, and    -   applying not only the session number and the respective session        key for the said one entity but also the third session key to        the respective security algorithm in the said one entity and the        processing means in order to produce the respective signature        and the respective result.

So that the said one entity is certain that the session whose executionis requested is validated by the third entity, the session numberretransmitted by the third entity and the session number transmitteddirectly by the processing means to the said one entity are compared inthe said one entity, and at least the step of applying in the said oneentity is executed only when the session numbers compared are identical.

The delegation can be transmitted to more than one entity. Thus at leastone other entity of the said at least two entities is delegated from thethird entity so that the session is executed only when the signaturesand the results produced according to the session number, the respectivesession keys and the third key are respectively identical.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the present invention willemerge more clearly from a reading of the following description ofseveral preferred embodiments of the invention with reference to thecorresponding accompanying drawings, in which:

FIG. 1 is a schematic block diagram of several electronic entities andof a data processing means of the smart card type in a telecommunicationnetwork for implementing the security method according to the invention;

FIG. 2 is an algorithm of steps of the security method with the dataprocessing means and two electronic entities according to a firstembodiment of the invention; and

FIG. 3 is an algorithm of steps of the security method with the dataprocessing means and a third electronic entity delegating to theprevious two entities, according to a second embodiment of theinvention.

DETAILED DESCRIPTION

FIG. 1 depicts a telecommunication network RT designating overall allthe telecommunication network types such as a radio telephone network,the switched telephone network, an integrated services digital networkISDN, a high-rate network such as an ATM network or the Internet, apacket transmission network etc. The network RT constitutes a means ofcommunication between a data processing means CA and various electronicentities, three of which are depicted, EX, EY and EZ.

By way of example to which reference will be made subsequently, the dataprocessing means is a controller, such as the microcontroller of a smartcard CA, in which a session must be initiated, which may be a task to beexecuted in the data processing means itself or an exchange of dataunits, such as messages, with at least one of the entities EX, EY andEZ. Thus the data processing means can be not only a smart card, alsoreferred to as a microcontroller card, but also any other portableelectronic object, such as an electronic assistant or organiser, anelectronic purse, a token or a pocket calculator.

An electronic entity, for example the entity EX or EY, is a serverdistant from the card CA, for example belonging to the issuer of thecard CA or in relationship with one of the applications implemented inthe card CA.

In a variant, the entities EX and EY are themselves smart cards housedin additional readers included in servers distant from the card CA sothat two administrators, possessors of the smart cards, enable a sessionthrough the smart card of a user.

The entity EZ can be a terminal TA accepting the smart card CA, such asa banking terminal, a point of sale terminal or a mobile radio telephoneterminal provided with an additional card reader, or a third server asis provided in the second embodiment described below.

According to a first embodiment of the method of the invention, theexecution of a session with the smart card CA is made secure under thecontrol of two entities EX and EY.

For example, the smart card CA is a card with a loyalty points accountissued by a fuel distribution company. After insertion in a terminal TAat a service station, as the entity EZ, the card CA is enabled to bedebited only by the two entities EX and EY so that the holder of thecard receives the article of his choice corresponding to a debiting ofpoints. The first entity EX is an article supplier server which simplyauthorises the card CA to be debited after recognition thereof. Thesecond entity EY is a server belonging to the fuel distribution companywhich verifies not only the identity of the card CA but also the pointsaccount contained therein in order to authorise the debiting of theaccount in the card CA. Thus the session consisting here of debiting theloyalty points account in the card CA is authorised only after theidentification of the card by the two entities EX and EY and theacceptance of the debiting by the entity EY, or in more global termsafter the acceptance of the execution of the “debiting of points”session by the two entities EX and EY.

According to another example, the possessor of the card CA must obtainthe authorisation of other two smart card possessors, as entities EX andEY, for example in order to access predetermined files in an Intranet.The “administering” cards EX and EY are then inserted in the readers ofterminals in the network in order to transmit to the card CA anacceptance or refusal of the session according to rights of access tothe predetermined files.

It is assumed in advance that the smart card CA is preferably proactiveand can thus itself trigger actions towards the outside world consistingin particular of the telecommunication network RT through the acceptingterminal TA, which is then transparent to these actions, although in avariant certain actions can be triggered by the accepting terminal TAitself. The card CA by nature has a privileged link with the entities EXand EY and contains in non-volatile memory EEPROM destination addressesADX and ADY of the entities EX and EY, such as their telephone numbersor their IP (Internet Protocol) addresses. The non-volatile memory ofthe card CA also contains public enciphering keys KPX and KPYrespectively associated with the entities EX and EY.

The security method according to the first embodiment shown in FIG. 2first of all comprises two sets of steps X1 to X9 and Y1 to Y9, whichare respectively associated with exchanges between the card CA and thefirst entity EX on the one hand and the card CA and the second entity EYon the other hand, and then final steps F9 to F15. Steps X1 to X9 beingrespectively identical to steps Y1 to Y9, the method is first of alldescribed in detail only for exchanges between the card CA and the firstentity EX.

As soon as the card CA decides to execute a session, for examplefollowing a request from the accepting terminal TA, the card CAinitiates an authentication of the card CA by the first entity EX, atstep X1. The authentication is conventional and consists essentially oftransmitting a random number by the first entity EX to the card CA andcomparing in the entity EX the results of the application of this randomnumber and of an authentication key both prestored in the card CA andthe entity EX, performed both in the card CA and the entity EX.Conversely, the card CA authenticates the entity EX. More completely ina variant, the authentication is mutual, that is to say theauthentication comprises an authentication of the card CA by the entityEX and an authentication of the entity EX by the card CA.

In a variant, the security method contains no authentication.

If after optional authentication the card CA receives no invalidationmessage, the card CA generates a session key KSX which may be random andassociates therewith a session number NSX at step X2. Then, after havingstored the key KSX and the number NSX in correspondence, the card CAprepares a message to be transmitted to the entity EX, containing therespective session number NSX and the respective session key KSX whichwere enciphered by means of the respective public enciphering key KPX.The enciphered message MEX thus formed is transmitted by the card CA tothe entity EX at step X3.

After deciphering of the message formulated according to a privatedeciphering key corresponding to the public card key KPX at step X4, theentity EX establishes first data DX in particular to indicate itsacceptance of the session to be executed, or where applicable itsrefusal, at step X5. Then the entity EX determines a signature SGXresulting from the application of the session number NSX and of thesession key KSX received to a first security algorithm ASX, at step X6.The entity EX constructs a control message CX which contains the sessionnumber NSX, the signature SGX=ASX (NSX, KSX) and the data DX and whichis transmitted to the card CA at step X7. The content of the controlmessage CX is preferably enciphered in a similar manner to that of themessage MEX.

In the card CA, after the transmission of the enciphered message MEX atstep X3, there is also determined a result REX of the application of thesession number NSX and of the session key KSX to the first securityalgorithm ASX, at step X8. The result REX is written in non-volatilememory in the card CA until it is read at step X9, in response to thecontrol message CX. At this step X9, the signature SGX received by thecard and corresponding to the session number NSX is compared with theresult REX stored in the card. If the signature SGX is different fromthe result REX, the session requested by the terminal TA with the cardCA is refused by the latter.

Otherwise, when the signature SGX is identical to the result REX, themethod passes to the final steps since steps Y1 to Y9 also result in astep Y9 according to which a second signature SGY transmitted by theentity EY is identical to a second result REY determined by the card CA.As is clear in FIG. 2, steps Y1 to Y9 are derived from the previouslydescribed steps X1 to X9, replacing the letter X with the letter Y. Thusthe second result REY results from the application in the card CA of asecond session number NSY and of a session key KSY, which may be random,both generated at step Y2 by the card CA, to a second security algorithmASY. The second signature SGY results from the application in the secondentity EY at step Y4 of the session number NSY and of the session keyKSY transmitted in enciphered form in a message MEY by the card CA atstep Y3, to the second security algorithm ASY. The second entity EYtransmits, at step Y7 also in a control message CY, preferablyenciphered, the number NSY and the signature SCY as well as second dataDY representing the acceptance of the execution of the session via theentity EY or a refusal thereof.

After identity of the first signature SGX and of the first result REX atstep X9 and identity of the second signature SGY and of the secondresult REY at step Y9, the card CA compares the data DX and DY at stepF9. If one or other of the data DX and DY represents a refusal, or ifone NSX or the other NSY of the session numbers retransmitted by theentities EX and EY is different from the number allocated at step X2 orY2, the session requested is not executed. Otherwise the data DX and DYrepresent an acceptance of the session corresponding to the numbersreceived NSX and NSY by the entities EX and EY and the method iscontinued by the execution of the session at step F10.

According to other variants of the first embodiment, the first sessionnumber NSX allocated at the data exchange between the first entity EXand the card CA and the second session number NSY allocated at the dataexchange between the card CA and the second entity EY are identical, andthe first and second security algorithms ASX and ASY are identical.

According to a first variant of final steps shown in short broken linesin FIG. 2, the smart card CA transmits respective acknowledgements ACKXand ACKY to the first and second entities EX and EY when both the firstsignature SGX is identical to the first result REX and the secondsignature SGY is identical to the second result REY, at steps X9 and Y9.Preferably the transmission of the first and second acknowledgementsACKX and ACKY occur rather after the final step F9, when the card CA hasdetected in the first and second data DX and DY an acceptance of thesession by the entities EX and EY. By virtue of these twoacknowledgements, the entities EX and EY each know that the other entityhas accepted the session. The session can be executed at the followingstep F10 as illustrated in FIG. 2, or in a variant previously to thetransmissions of the acknowledgements ACKX and ACKY.

According to a second variant of final steps, after the finding of theidentities of signature and result at steps X9 and Y9, preferably afterthe detection of an acceptance of the session by the entities EX and EY,the card CA produces, at a step F11, a word ACK representing the sessionto be executed at a step F10. In this regard, the session can beexecuted at step F10 before the transmission of the word ACK at step F11as illustrated in FIG. 2, or in a variant after step F11.

More precisely, according to this second variant, the card CA produces afirst word signature SAX resulting from the application of therepresentative word ACK and of the first session key KSX to the firstsecurity algorithm ASX, and a second word signature SAY resulting fromthe application of the representative word ACK and of the second sessionkey KSY to the second security algorithm ASY. The card CA encapsulatesthe word ACK and the word signatures SAX and SAY in a message AY inorder to transmit it to one of the entities, for example the secondentity EY, at a step F12.

The second entity EY verifies the correspondence between the receivedword ACK representing the session and the second respective wordsignature SAY according to the respective session key KSY which has beenreceived and stored in the entity EY at step Y4, by applying thereceived word ACK and the key KSY to the second algorithm ASY so as toproduce a result which is compared with the second received signatureSAY, at a step F13. If this comparison is positive, that is to say ifthe received word ACK corresponds to the signature SAY, the secondentity EY transmits a message AX containing the word ACK representingthe session and the other signature, that is to say the first wordsignature SAX=ASK (ACK; KSY), to the other entity EX at a step F14. Onreception of the message AX, the first entity EX verifies thecorrespondence between the representative word ACK and the firstreceived word signature SAX according to the respective session key KSXwhich has been received and stored in the entity EX at step X4. Thisverification consists of applying the received word ACK and the firstsession key KSX to the first security algorithm ASX and comparing theresult produced by this algorithm with the received signature SAX at astep F15.

If at step F13 the entity EY finds a lack of correspondence between therepresentative session word ACK and the second word signature SAY, theentity EY ignores the result of the session executed and does nottransmit the message AX to the entity EX or transmits a negativeacknowledgement message to the entity EX; in a variant, the entity EYalso proceeds with a cancellation of the session when it is still to beexecuted in the card CA via the terminal TA. Likewise, when the firstentity EX finds a lack of correspondence between the representativesession word ACK and the first word signature SAX, the entity EX ignoresthe result of the executed session and preferably signals it to theentity EX; in a variant, the entity EX also proceeds with a cancellationof the session in the card CA when it is to be executed.

In a variant, the acknowledgement message ACKX and ACKY, and/or themessages AX and AY, are enciphered.

Although the first embodiment has been described with two entities EXand EY, the invention also encompasses embodiments with more than twoentities which must each give its acceptance to the card CA according tosteps X1 to X9, Y1 to Y9 in order to authorise execution of the session.In particular, for the second variant shown at the bottom of FIG. 2,step F11 produces as many word signatures SAX, SAY as there are entitiesEX, EY, and each of these entities performs a step F13, F15 during whichit verifies the correspondence between the word ACK representing thesession and the respective word signature SAX, SAY according to therespective session key KSX, KSY, and so on until the last entity.

According to a second embodiment of the security method according to theinvention, a third electronic entity EZ intervenes. When the card CAdecides to execute a predetermined session, it routinely interrogatesthe third entity EZ which does not have enough information to decidewhether or not it accepts the requested session; the entity EZ thendelegates this decision for a predetermined period to the first andsecond entities EX and EY by transmitting to them first and seconddelegation information IDX and IDY respectively.

According to a complementary variant, if the commands executed in thesession of step F10 require the intervention of the entity EZ and if theentity EZ cannot or does not wish to intervene in this interactiveexchange, the delegation enables the entity EZ to indicate to the cardCA that the entity EX, EY which has received the delegation has theright to act on behalf of the entity EZ.

For example, as shown in FIG. 1, the third entity EZ, the delegator, isa server of a bank which, during an annual holiday period, allows creditto the card owner CA, and subsequently confides in a first server EX ofa commercial site connected to the Internet and presenting products tobe purchased and also in a second server EY of a product supplier. Whenthe user, the delegatee, decides, by means of its own computer terminalTA connected to the network RT and provided with an additional cardreader in which the card CA is inserted, to purchase a product from theserver EX, this transaction is triggered by the bank server EZ which hasverified that the account corresponding to the smart card CA has anauthorised credit and which has the transaction relayed by the serversEX and EY, the delegates, in so far as the latter have received avalidation in the form of a key KSZ supplied by the card CA andretransmitted by the server EZ, as will be seen below.

It is assumed in this second embodiment that the entities EX and EY havealready had knowledge of the delegation transmitted by the third entityEZ.

As is clear by comparing FIGS. 2 and 3, the second embodiment of themethod according to the invention first of all comprises steps Z1 to Z7relating to exchanges of data between the third entity EZ and the smartcard CA. In a similar manner to the first embodiment, the card CAcontains in non-volatile memory the destination addresses ADX, DAY andADZ of the entities EX, EY and EZ as well as public enciphering keysKPX, KPY and KPZ associated with these entities.

At the first step Z1, following a session execution request by the cardCA transmitted to the entity EZ, the entity EZ authenticates the cardCA, or in a variant the entity EZ and the card CA authenticate eachother mutually.

In a variant, the second embodiment of the security method contains noauthentication.

After optional authentication, the entity EZ supplies the first andsecond items of delegation information IDX and IDY to the card CA. Eachof the first and second items of delegation information contains forexample the address ADX, ADY, or other delegate identifier, of theentity EX, EY, and the number of authorities required for executing thesession, that is to say the number of entities such as the entities EXand EY whose acceptance is required for executing the session. Thus, atstep Z2, the third entity EZ transmits the first and second items ofdelegation information IDX and IDY as well as the source address ADZ ofthe entity EZ to the card CA in the form of a message which is signedwith the private key of the third entity EZ corresponding to the publickey KPZ, and then enciphered with the public key KPCA of the card CA.After deciphering, signature verification and storage of the informationIDX and IY at step Z3, the card CA generates a session number NS as wellas three session keys KSX, KSY and KSZ, which may be random, andassociates them respectively with the entities EX, EY and EZ incorrespondence with the session number NS, at step Z4. These fourparameters NS, KSX, KSY and KSZ are stored in the card in order to servein the subsequent steps.

At the following step Z5, the card CA enciphers the session number NSand the third session key KSZ with the enciphering key KPZ in order totransmit them in an enciphered message MEZ to the third entity EZ. Afterdeciphering of the message MEZ and storage of the number NS and of thesession key KSZ at step Z6, the entity EZ establishes two messages MZXand MZY transmitted respectively to the entities EX and EY. The firstmessage MZX comprises the session number NS and the third session keyKSZ and the destination address ADX which are enciphered by means of thepublic key KPX of the first entity EX. The second message MZY alsocomprises the number NS, the key KSZ and the address ADY which areenciphered by means of the public key KPY of the second entity EY.Messages MZX and MZY are respectively received by the entities EX and EYin order to be deciphered therein by means of their private encipheringkeys and to be stored therein at following steps Z8X and Z8Y.

In parallel with steps Z4 to Z7, the card CA performs steps X1 to X4 andY1 to Y4, substantially identical to those already described withreference to FIG. 2, in response to the delegation information IDX andIDY received at step Z3, so as to authenticate the card CA by means ofthe entities EX and EY delegated by the entity EZ and to transmitenciphered messages MEX[NS, KSX] and MEY[NS, KSY] by the card CA to theentities EX and EY and to decipher these messages at steps X4 and Y4.

Then, at a step Z9X, Z9Y in the entity EX, EY, the session number NSstored at step Z8X, Z8Y and transmitted by the third entity EZ iscompared with the session number NS and transmitted by the card CA anddeciphered at step X4, Y4, by analogy with the comparison of the sessionnumber received and stored NSX, NSY at step F9. If the session numbersare different, the entity EX refuses the requested session. Otherwisethe two session numbers are identical and data DX, DY representing anacceptance of the session by the delegated entity EX, EY at the stepwhere X5, Y5 are established. The method is continued with steps X6Z andX7Z, Y6Z and Y7Z replacing respectively steps X6 and X7, Y6 and Y7, andbeing distinguished from these by the fact that the signature SGXZ, SGYZis determined by applying the session number NS validated at theprevious step Z9X, Z9Y, the session key KSX, KSY received and decipheredat step X4 and the third session KSZ received, deciphered and stored atstep Z8X, Z8Y, to the security algorithm ASX, ASY. The session numberNS, the signature SGXZ, SGYZ and the data DX, DY are preferablyenciphered and encapsulated in a message CXZ, CYZ which is transmittedto the card CA.

In parallel to steps X4 to X7Z, Y4 to Y7Z, a result REXZ, REYZ isdetermined in the card at a step X8Z, Y8Z replacing step X8, Y8, byapplying the session number NS, the key KSX, KSY and the third key KSZto the security algorithm ASX, ASY.

The following step X9Z, Y9Z in the card CA compares the signature SGXZ,SGYZ with the result REXZ, REYZ so as to pass to the final step F9 whenthe identities SGXZ=REXZ and SGYZ=REYZ are verified.

By virtue of the transmission of the third key KSZ by the card CAthrough the third entity EZ and the transmission of the keys KSX and KSYby the card CA directly to the entities EX and EY, the transmission ofthe signatures SGXZ and SGYZ dependent on these two pairs of keys withacceptance data DX and DY to the card CA ensure that the entities EX andEY have recovered the delegation of the entity EZ and are authorised togive the instruction to execute the number session NS by delegation.

According to a third embodiment combining the first and secondembodiments, only one of the entities EX and EY, for example the firstentity EX, is delegated by the third entity EZ. A session is executedonly when the card CA has received the acceptance of the entity EX bydelegation from the entity EZ and the acceptance of the entity EYindependent of the entity EZ.

For the third embodiment, the left-hand part of the algorithm in FIG. 3with respect to the card CA, that is to say steps Z1 to Z7 omittingIDY(ADY), KSY and KPY and steps Z8X to X9Z, is preserved, and theright-hand part of the algorithm in FIG. 3 concerning the relationshipswith the entity EY is replaced by the steps Y1 to Y9 to the right inFIG. 2, in order finally to compare the signature SGXZ with the resultREXZ and the signature SGY with the result REY at steps X9Z and Y9before reading the received data DX and DY in the card CA at step F9.

1. A method for making secure the execution of a session with a dataprocessing means under the command of at least two electronic entities,comprising the following steps: transmitting session numbers and sessionkeys from the data processing means to the entities, applying arespective session number and a respective session key to a respectivesecurity algorithm in the data processing means and the respectiveentities to produce a respective result and a respective signature,transmitting the respective session number and the respective signaturefrom the respective entities to the data processing means, and executinga session corresponding to the session numbers retransmitted from thedata processing means when the signatures are respectively identical tothe results.
 2. A method according to claim 1, wherein the respectiveresults are written in memory in the data processing means respectivelyin correspondence with the respective session numbers to be transmittedto the entities and are read in correspondence with the respectivesession numbers transmitted by the entities to the data processing meansbefore being compared with the respective signatures.
 3. A methodaccording to claim 1 wherein each of the entities transmits to theprocessing means respective data to the data processing means with therespective session number and the respective signature, and the sessionis executed if the data processing means detects in each of the data anacceptance of the session by the respective entity.
 4. A methodaccording to claim 1, wherein before being transmitted from the dataprocessing means, the respective session number and the respectivesession key are enciphered by a respective enciphering algorithm with arespective public key for each of the entities.
 5. A method according toclaim 1, wherein the data processing means transmits respectiveacknowledgements to the respective entities when the signatures arerespectively identical to the results.
 6. A method according to claim 1,wherein the data processing means produces a word representing thesession when the session is to be executed, generates respective wordsignatures each resulting from application of said word and a respectivesession key to the respective security algorithm, and transmits saidword and the word signatures to one of the entities to verify thecorrespondence between said word and the respective word signatureaccording to the respective session key and, when there is acorrespondence, to transmit said word and the other respective wordsignatures to another entity, which verifies the correspondence betweensaid word and the respective word signature according to the respectivesession key.
 7. A method according to claim 1, further including aninitial authentication of the data processing means by the entitiesand/or vice-versa.
 8. A method according to claim 1, further includingthe following steps: transmitting respective delegation informationrelating to said two entities from a third electronic entity to the dataprocessing means, transmitting a session number and a third session keyfrom the data processing means to the third entity, retransmitting thesession number and the third session key by the third entity to one ofsaid two entities, and applying the session number, the respectivesession key for said one entity and the third session key to therespective security algorithm in said one entity and the data processingmeans to produce a respective signature and a respective result.
 9. Amethod according to claim 8, wherein the delegation information issigned with a private key of the third entity and enciphered with apublic key of the data processing means.
 10. A method according to claim8, wherein the session number retransmitted by the third entity and thesession number transmitted directly by the data processing means to saidone entity are compared in said one entity, and at least the step ofapplying in said one entity is executed only when the compared sessionnumbers are identical.
 11. A method according to claim 8, wherein beforebeing transmitted and retransmitted, the session number and the thirdsession key are enciphered with a third public key of the third entity,and with a public key of said one entity.
 12. A method according toclaim 8, wherein at least one other entity of said at least two entitiesis delegated by the third entity so that the session is executed onlywhen the signatures and the results produced according to the sessionnumber, the respective session keys and the third session are identical.13. A method according to claim 1, wherein at least one of the dataprocessing means and the entities is a smart card.
 14. A method forconducting a secure session between a smart card and at least oneentity, comprising the following steps: transmitting session numbers andsession keys from the smart card to said one entity and at least oneother entity, applying a received session number and a received sessionkey to a security algorithm in each of said one entity and said otherentity, to produce respective signatures, applying the transmittedsession numbers and session keys to respective security algorithms insaid smart card, to produce respective results, transmitting the sessionnumbers and the signatures produced in the respective entities to thesmart card, comparing the received signatures with the respectiveresults in the smart card, and executing a session corresponding to thesession number between the smart card and said one entity when thesignatures are identical to the respective results.
 15. A methodaccording to claim 14 wherein each of said entities transmits respectivedata to the smart card with its respective session number and signature,and the session is executed if the smart card detects an acceptance ofthe session in the data received from each entity.
 16. A methodaccording to claim 14, wherein the session numbers and the session keysare encrypted by a respective encryption algorithm with a respectivepublic key for each of the entities, before being transmitted from thesmart card.
 17. A method according to claim 14, wherein the smart cardtransmits acknowledgements to the respective entities when thesignatures are identical to the respective results.
 18. A methodaccording to claim 14, further including the following steps:transmitting respective delegation information relating to said oneentity and said other entity from a third entity to the smart card,transmitting a session number and a third session key from the smartcard to the third entity, retransmitting the session number and thethird session key by the third entity to said one entity, and applyingthe session number, the session key for said one entity and the thirdsession key to the security algorithm in said one entity and the smartcard to produce a respective signature and a respective result.
 19. Amethod according to claim 18, wherein the delegation information issigned with a private key of the third entity and enciphered with apublic key of the smart card.
 20. A method according to claim 18,wherein the session number retransmitted by the third entity and thesession number transmitted directly by the smart card to said one entityare compared in said one entity, and the step of applying in said oneentity is executed only when the compared session numbers are identical.